This invention relates to a method and a device for attaching a semiconductor die to a substrate in a manner that an electrical short to the PN junction of the semiconductor die is prevented.
Die attaching forms an integral part of a semiconductor chip packaging process. Die attaching or die bonding is the process of mounting a semiconductor die or chip onto a substrate and this process is well established in the field of semiconductor device manufacturing.
One purpose of the die attaching process is to create a strong physical bond between the die and the substrate of the package. However, it also serves to provide either an electrical conducting or insulating contact between the die and the substrate. There are different known die attaching methods, such as eutectic die attachment, soldering die attachment and adhesive die attachment. The choice among the different die attaching methods in a particular case depends on factors like the size of the semiconductor die, the substrate material and the operating conditions of the final device.
A good die attaching process provides for a die attach which is strong and does not loosen or deteriorate in quality over time. The die attaching process should also be productive and economical. This makes the adhesive die attaching process popular as this process is very robust and can provide for a high throughput using automated equipments.
In the process of adhesive die attachment using conductive adhesive according to the prior art, a small amount of conducting adhesive is dotted or dispensed onto a substrate 102 as shown in FIG. 1. A collet then picks up a semiconductor die 100 from a wafer ring and brings the semiconductor die 100 directly above the adhesive on the substrate 102. The collet then places the die 100 onto the adhesive in a downward manner, exerting a slight pressure to press the semiconductor die 100 down firmly onto the adhesive. As a result of this action, the excess adhesive 101 wets up the sides of the semiconductor die 100, forming a fillet 101 around the lower region of the die 100. The adhesive is subsequently cured by heating in order to harden the same and thereby to provide a firm bond between the die 100 and the substrate 102 so that the semiconductor die 100 is reliably held in place and electrically connected with its bottom surface electrode to the respective contact surface of the substrate 102.
Many semiconductor dice, in particular LEDs (Light Emitting Diodes), laser diodes and photodetectors have an N-doped 103 and a P-doped 104 portion as shown in FIG. 2 (or vice versa). The PN junction 105 of the N-doped 103 and P-doped 104 portions of the semiconductor die may be grown very near, e.g. less than 50 μm, to the base of the die. In fact, there is a tendency to grown the PN junction as close to the base of the die as possible, since a low PN junction provides the device with desirable characteristics like high light output and good heat conductivity. However, when the excess adhesive wets up the sides of the die and thereby forms a fillet 101 around the lower region of the die 100 which contains the PN junction 105, an electrical short circuit occurs at the PN junction 105 as it is short-circuited by the conductive adhesive, and the die 100 will become useless.
One known solution to overcome this problem is to reduce the amount of adhesive used for the attaching process. For example, in case of a LED die with a size of 0.25 mm square, the diameter of the conductive adhesive dot dotted onto the substrate must be kept less than 0.2 mm to prevent wetting up of the sides of the die and thereby shorting of the PN junction. However, reducing the diameter and thereby the amount of adhesive dot leads to deterioration of the overall mechanical strength of the bond of the die to the substrate and thus to reliability problems, especially if the device is operated in a cyclical temperature environment. In particular, with a reduced amount of adhesive, the lateral strength of the bond is significantly reduced. Furthermore, the deposition of a small and accurate amount of adhesive makes the die attachment process very difficult to control.
Another known solution to overcome the above described problem is to use eutectic die attach instead of adhesive die attach. The eutectic die attaching method is adopted by many companies due to the strong bonding and good heat dissipation properties of the eutectic die attach.
However, the automation of the process for eutectic bonding is not as easy as the automation of the adhesive die attaching method. Further, the adhesive die attaching method is cheaper and requires a lower bonding temperature than the eutectic die attaching method.
Still another known solution for the above described problem uses a method in which the conductive adhesive is dispensed and partially cured on the wafer before sawing the same into individual dice. Typically, a silver epoxy adhesive is deposited onto the wafer and the epoxy is pre-cured before the wafer is cut into individual dice for attachment to the substrates. Furthermore, in this method, the conductive adhesive is formed by an epoxy material to which silver balls as electrically conductive fillers are added. The use of ball-formed fillers containing adhesive is necessary in this method, since only such ball-formed particles enable a uniform, even dispersion of the adhesive over the entire surface of the wafer.
However, one disadvantage of this method is that it may not be suitable in cases where the individual dice still have to undergo further chemical and/or heat treatments before they are attached or bonded to the substrate, since such an additional treatment could alter essential bonding characteristics of the adhesive and thereby render the adhesive not useable for the later bonding purposes, or eventually even wash away the adhesive from the surface of the die. Therefore, this method cannot be used for example in cases where a post-saw etching process is needed or in cases of non-rectangular dice, like a shaped-die with trapezoid cross-section, since the production of such dice may require a chemical treatment of the individual dice after sawing the wafer into the said individual dice.